Local oscillators are used in radio-frequency (RF) transmitters and receivers and in other applications to produce an adjustable-carrier or heterodyning frequency. An oscillator with an adjustable frequency enables a transceiver to communicate over a chosen channel. The local oscillator is generally implemented as a voltage-controlled oscillator (VCO) so that its frequency can be adjusted by a variable applied voltage. VCOs are generally implemented with a resonant tank circuit using a fixed inductor and a capacitor that is controlled with a voltage. Varactor diodes can be used to provide a voltage-controlled capacitance, which are back-biased semiconductor diodes wherein their junction depth and internal stored charge in the semiconductor body depend nonlinearly on the applied reverse voltage.
In order to reduce the sensitivity of the frequency of a local oscillator designed as a VCO to noise superimposed on an analog control signal, the VCO tuning range controlled by the analog control signal is constrained to be small. VCOs in communication applications require generation of a precision frequency so that adjacent-channel interference can be avoided. So that the total tuning range of the VCO can be sufficiently wide, which is required in applications wherein operation over a plurality of communication bands is required, the frequency of the VCO is generally controlled by both an analog varactor diode for fine frequency adjustment and a bank of digitally selectable capacitors for wider frequency adjustment. A digital control circuit is needed to select the correct configuration of switched capacitors from a capacitor bank in order to center the VCO frequency around the narrow tuning range of the varactor diode. The control circuit should be compact to obtain low cost, and for frequency hopping applications, it must be able to rapidly tune the capacitor bank, for example in less than 200 μs.
Control circuits of the prior art using digital processes for selection of resonant tank circuit tuning capacitors require extended numerical computation with substantial implementation and manufacturing complexity for their implementation. Digital processes that require extended numerical computation such as multiplication or division of two variables are often implemented with complex digital signal processing techniques or with custom digital circuits with a substantial number of logic gates. Either approach uses significant die area for the circuit, resulting in a costly or otherwise impractical design for a VCO controller targeted at a broad commercial market. In addition, the extensive computation required for capacitor selection adversely affects the time required for a precision change in oscillator frequency. Recognizing that applications of voltage-controlled oscillators include cellular telephones and portable radio transceivers, both of which serve large, competitive markets that include rapid frequency-hopping features, a low-cost VCO controller with a quickly adjustable-frequency would provide a competitive advantage.
The main limitations of the prior art circuits are devices implemented with complex or slow processes to control the frequency of an adjustable-frequency oscillator. The prior art approaches use processes that employ extended arithmetic operations and extensive numerical computation. A need thus exists for an apparatus and method to quickly select capacitors from a capacitor bank to control the frequency of an adjustable-frequency oscillator that can be implemented with relatively simple and low-cost digital logic.